The present invention relates generally to optical communications, and more particularly, to a method for compensation of nonlinear impairments in dispersion-managed optical fiber links using phase conjugation and equivalent optical link.
In order to increase the fiber capacity two things have to be done: 1) Increase modulation order and 2) Reduce channel spacing on WDM systems. For 1, high optical-to-signal-noise-ratio OSNR is required, which translates into high optical power per channel. High power means high nonlinearity, which distorts the optical signal. This is called intra-channel nonlinear distortion. For 2, reducing channel spacing yields to increased nonlinear cross-talk between wavelength-division multiplexing WDM channels. This is called inter-channel nonlinear distortion.
From the above, fiber capacity is limited by the action of fiber nonlinearities. This invention proposes a method to compensate the nonlinear distortion, both intra- and inter-channel. Nonlinearity compensation allows increase either transmission capacity (for a give transmission distance) or transmission distance (for a given transmission capacity).
A number of techniques have been used to mitigate and compensate fiber nonlinearity. In particular, two are Mid-span Optical phase conjugation and Digital Backward Propagation (hereafter BP).
Mid-Span Optical Phase Conjugation
This method consists of performing optical phase conjugation in the middle of the transmission link. If the link has certain symmetry properties, the second half of the link compensates the nonlinear distortion created in the first half. Optical phase conjugation can be implemented in the optical domain or in the electrical domain. This technique presents the following limitations:                i) Typical transmission link are non-symmetric        ii) Implementation of optical phase conjugation requires cumbersome optical set-up with nonlinear optics involved.        
Digital Backward Propagation (or Back-Propagation) (Hereafter BP).
This technique compensates fiber impairments (including nonlinearity) in the digital domain. This technique involves the following steps:                a. Coherent detection of the optical signal: This step allows to recover both amplitude and phase.        b. Analog-to-digital conversion: This step creates a digital version of the detected signal.        c. Signal reconstruction: This step creates a digital version of the optical field at the end of the fiber.        d. Back-propagation: This step takes the signal in c. and simulates optical propagation with negative parameters (i.e. backwards). This step involves:                    d.1. Knowledge of the fiber characteristics, namely: Dispersion parameter(s), nonlinear coefficient, distance(s) between amplifiers, gain of the amplifiers, fiber loss, input power.            d.2. Solve the BP-equations with the above parameters. This is typically done using the so-called split step method. This method consists on dividing the transmission in multiple steps. The amount of steps depends on how fast the optical field changes along the link.                        
Accordingly, there is a need for an improved method for compensation of nonlinear impairments in dispersion-managed optical fiber links.